8.11.2 Table Locking Issues
InnoDB tables use row-level locking so that multiple sessions and applications can read from and write to the same table simultaneously, without making each other wait or producing inconsistent results. For this storage engine, avoid using the
LOCK TABLES statement, because it does not offer any extra protection, but instead reduces concurrency. The automatic row-level locking makes these tables suitable for your busiest databases with your most important data, while also simplifying application logic since you do not need to lock and unlock tables. Consequently, the
InnoDB storage engine is the default in MySQL.
MySQL uses table locking (instead of page, row, or column locking) for all storage engines except
InnoDB. The locking operations themselves do not have much overhead. But because only one session can write to a table at any one time, for best performance with these other storage engines, use them primarily for tables that are queried often and rarely inserted into or updated.
When choosing whether to create a table using
InnoDB or a different storage engine, keep in mind the following disadvantages of table locking:
Table locking enables many sessions to read from a table at the same time, but if a session wants to write to a table, it must first get exclusive access, meaning it might have to wait for other sessions to finish with the table first. During the update, all other sessions that want to access this particular table must wait until the update is done.
Table locking causes problems when a session is waiting because the disk is full and free space needs to become available before the session can proceed. In this case, all sessions that want to access the problem table are also put in a waiting state until more disk space is made available.
SELECTstatement that takes a long time to run prevents other sessions from updating the table in the meantime, making the other sessions appear slow or unresponsive. While a session is waiting to get exclusive access to the table for updates, other sessions that issue
SELECTstatements will queue up behind it, reducing concurrency even for read-only sessions.
The following items describe some ways to avoid or reduce contention caused by table locking:
Consider switching the table to the
InnoDBstorage engine, either using
CREATE TABLE ... ENGINE=INNODBduring setup, or using
ALTER TABLE ... ENGINE=INNODBfor an existing table. See Chapter 14, The InnoDB Storage Engine for more details about this storage engine.
SELECTstatements to run faster so that they lock tables for a shorter time. You might have to create some summary tables to do this.
Start mysqld with
--low-priority-updates. For storage engines that use only table-level locking (such as
MERGE), this gives all statements that update (modify) a table lower priority than
SELECTstatements. In this case, the second
SELECTstatement in the preceding scenario would execute before the
UPDATEstatement, and would not wait for the first
To specify that all updates issued in a specific connection should be done with low priority, set the
low_priority_updatesserver system variable equal to 1.
Start mysqld with a low value for the
max_write_lock_countsystem variable to force MySQL to temporarily elevate the priority of all
SELECTstatements that are waiting for a table after a specific number of inserts to the table occur. This permits
READlocks after a certain number of
Splitting table contents into separate tables may help, by allowing queries to run against columns in one table, while updates are confined to columns in a different table.
You could change the locking code in
mysys/thr_lock.cto use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.